METHOD AND BENDING DEVICE FOR BENDING FLAT METAL WORKPIECES

Abstract

The invention relates to a method and device for bending flat metal workpieces, especially steel sheets, by bending freely over edges along a bending line with a straight, linear movement of the tool, wherein the bending edges are arranged parallel to the longitudinal direction of the sheet and the tool is displaced along the width of the metal sheet, characterized by the steps: inserting a punch between the bending edges of a die, holding the punch at the lowest point of the bend, applying compressive stress in order to level out the stresses in the shaping zone produced by the bending and incrementally superimposing the tensile stress with compressive stress, that is effective along the bending line in the direction of the sheet width of the bent workpiece, wherein the tool producing the compressive stress is moved along the bending line in the direction of the sheet width. In the bending device between the die assemblies below the punch, a carriage is arranged that can be displaced in the direction of the bending axis (i.e. in the sheet width) with a nip roller being effective in the punch axis, which has a groove in the plane of intersection with the punch axis.

Description

The invention relates to a method for bending flat metal workpieces, in particular sheet steel, by bending freely, also referred to as free-bending or air-bending, over edges along a bending line with a straight linear movement of the tool.

The shaping is performed according to the invention by

a) inserting a punch between the bending edges of a forming die,

b) holding the punch at the lowest point of the bend,

c) applying compressive stress to superimpose the tensile stress in the shaping zone,

d) incrementally superimposing the tensile stress in the shaping zone with compressive stress along the bending line of the workpiece to be bent.

The invention also relates to a device for performing the method according to the above paragraph, consisting of a forming die with two parallel die assemblies for holding the bending edges and a punch arranged above the forming die, wherein the die assemblies can be interchanged for setting different die widths and underneath the punch a tool carriage is arranged which can be moved in the direction of the bending line and comprises a nip roller which is aligned and adjustable against the workpiece to be bent.

The present invention relates to bending methods standardised by DIN 8586 and in particular to the free bending of flat metal workpieces. In known bending methods, such as for example according to EP 0 341 211 A2, metal sheets are bent by means of a bending device, which comprises a punch as well as a forming die with an adjustable base. The punch penetrates according to the desired bending angle up to a previously defined penetration depth while simultaneously pushing the die base along the bending line.

It has been established in practice that variations often occur in the bending angle. This has been traced back to the fact that the sheet metal sides spring back by an undefined amount as soon as the punch has been released.

The undesirable springing back resulted in the development of an adaptive bending method according to DE 694 01 594, in which a correction value is calculated using a bending formula, in which the bending angle is established during the bending process by means of tracer pins, which are arranged laterally during the bending process on the workpiece, and by applying the bending formula a correction value is determined for the end position of the punch.

The aforementioned methods are complicated in practice and do not allow universal applications on any shapes of workpieces and bending angles. Therefore, a simple method has been sought in which the bending angle can be adjusted precisely and which with high precision makes it possible to obtain reproducible values for the geometry of the bent workpiece. In addition, the bending angle should be variable locally in relation to the width of the metal sheet and thus also the springing back behaviour should be adjustable locally. Furthermore, a bending device suitable for the new method should be structured to be simpler and should be able to be used more flexibly.

The solution to these problems consists according to the invention essentially in that the stresses created by the bending process in the shaping zone are superimposed by a compressive stress in locally and sequentially consecutive steps, i.e. incrementally. In this case it is assumed that maximum tensile stress is reached in a shaping zone displaced into the bend sides, so that here too the superimposition of stress has to be performed with the applied compressive stress in this area of the shaping zone. By controlling this process a localised change of the bending angle and springing back behaviour should be made possible in the direction of the sheet metal parts.

In practice, the device according to the invention therefore provides a pressure roller, in particular a steel roller with a preferably V-shaped groove, annular groove or with an elastomer coating underneath the punch. By means of the pressure rollers compressive stresses in the pulling area of the shaping zone are transmitted along the bending line, which on the one hand do not exceed the value of the flow beginning (R_{P 0,2}) and on the other hand change the stressed state of the bending part locally and in time, so that afterwards the springing back can be compensated for by more than 50%. For this if necessary a bending sensor is driven along in front of the pressure roller according to the path of the bending line on the workpiece and afterwards the advance of the nip roller is determined by means of a pressure control.

In principle, it is known from other shaping methods to reduce the springing back of the sheet metal by means of compressive stress superimposition. For example DE 196 08 985 A1 shows a pulling method with a divided pressure cushion in which the hold-down plate can be driven in the manner of a counter-pulling process during the shaping. However, specific steps for compensating the springing back during sheet bending cannot be derived therefrom.

In the following, the invention is explained in more detail with reference to several exemplary embodiments. The initial basis of the invention is the already mentioned free bending, in which according to FIG. 1 a punch is pushed from above onto a metal sheet. The metal sheet lies horizontally on a forming die, which is provided with two parallel die assemblies for the bending edges. If the punch is pushed with its punch tip into the forming die the metal sheet is bent downwards into the forming die, being supported laterally on the radii of the forming die edges. Hereby on the inner side of the metal sheet in the region of the shaping zone compressive stresses are created, whilst on the outside of the metal sheet in the region of the shaping zone there are tensile stresses. The metal sheet now has in the loaded state a bending angle α₁. It consists of the lateral bending sides, which run towards the lower punch bending edge.

It is in fact known that the maximum level of tensile stresses is along the bending line, however according to the inventor's findings it is important to provide compensation by superimposing the tensile stress with compressive stress not only in this area.

With a nip roller with a V-shaped groove or annular groove the compressive stress is created by the groove surface which presses laterally from the outside against the radius area. By means of this measure in the shaping zone the compressive stress is compensated for, which is sufficient to be able to remove the bent workpiece after release by the punch as a ready-to-use part.

In principle, the nip roller can also have different forms, such as for example that of a combination of an annular and V-groove or can have a spherical surface such as for example with a barrel. It is also possible to provide the nip roller with an elastomer coating.

It is advantageous, if at a distance in front of the nip roller a bending angle sensor is inserted between the bending edges. Then the advance of the nip roller can be pressure controlled depending on the measurement of the sensor signal but also on the compressive force to be set. However, it is also possible to move the sensor from the outside up to the bending edges, if this is expedient in consideration of the local conditions.

EXAMPLE 1

By means of the advance speed the superimposition of stresses is influenced in terms of time. Investigations have shown that good results can be obtained with a constant and relatively slow advance speed.

EXAMPLE 2

The geometry of the V-groove is such that the shaping zone is reached in the bending sides. In this case it is important that there is contact between the surface in the region of the groove and the bending part even outside the radius area.

EXAMPLE 3

The groove width should preferably be greater than 2× the bending radius or curve shape of the punch, so that with a V-shape the contact surface is as large as possible with the pressure roller along the bending line.

EXAMPLE 4

To improve the existing contact in the shaping zone at a constant working width, the method according to the invention can be performed with a variable application of pressure. This is a great advantage mainly in the case of metal sheets with different material properties or thicknesses.

Claims

1. Method for bending flat metal workpieces, in particular steel sheets, by bending freely over edges along a bending line with straight linear tool movement, wherein the bending edges are arranged parallel to the longitudinal direction of the sheet and the tool moves along the sheet width, characterised by the following steps

a) inserting a punch between the bending edges of a forming die,

b) holding the punch at the lowest point of the bend,

c) applying compressive stress to compensate for the stresses in the shaping zone produced by bending

d) incremental superimposition of the tensile stress by compressive stress, which is performed along the bending line in the direction of the sheet width of the bent workpiece, whereby the tool generating the compressive stress is moved along the bending line in the direction of the sheet width.

2. Method according to claim 1, characterised in that the superimposition of the tensile stress in the shaping zone is performed by an incrementally generated compressive stress between the bending edge and bending line, and the bending angle is influenced (adjusted) variably locally over the width of the metal sheet.

3. Method according to one of the preceding claims, characterised in that the incremental superimposition of the tensile stress with compressive stress is performed by surface pressure, which is in the elastic range i.e. below the yield stress limit RP 0,2 of the respective material.

4. Method according to one of the preceding claims, characterised in that the compressive stress is generated incrementally with an increasing and decreasing stress peak in the region of the maximum tensile stress between the bending edge and bending line.

5. Bending device for performing the method according to claim 1, consisting of a forming die with two parallel die assemblies and a punch arranged above the die assemblies, which punch can be inserted into the intermediate space between the bending edges, characterised in that between the die assemblies underneath the punch a carriage with a nip roller effective in the punch axis is arranged which can be moved in the direction of the bending axis (i.e. in the sheet width), which nip roller comprises a groove in the section plane with the punch axis.

6. Bending device according to claim 5, characterised in that a bending angle sensor can be inserted at a distance in front of the nip roller between the bending edges and the nip roller is advanced according to the sensor signal measurement.

7. Bending device for performing the method according to claim 1, characterised in that the groove has a V-shape, a radius.

8. Bending device according to one of the preceding claims, characterised in that the forming die comprises interchangeable and rotatable die assemblies and in this way different die widths can be adjusted.

9. Bending device according to one of the preceding claims, characterised in that the application of compressive stresses is performed incrementally by means of the nip roller along the bending line.

10. Bending device according to one of the preceding claims, characterised in that the advancing speed for the movement of the nip roller along the bending line of the bent workpiece is controlled.

11. Bending device according to one of the preceding claims, characterised in that the depth of the V-groove in the nip roller is measured such that the contact area of the selected groove (V-groove, annular groove) extends with the external wall of the metal sheet into the pulling area of the shaping zone of the metal sheet.

12. Bending device according to one of the preceding claims, characterised in that the groove width of the V-groove is adjusted to the bending radius of the metal sheet, so that a large contact area is created with the nip roller over the entire width of the shaping zone.